Mount Thielsen is a shield volcano built on a broad platform of older lava. Glaciers have removed much of the volcano, exposing a jagged central plug that is flanked colorful pyroclastic rocks. Mount Thielsen is about 290,000 years old.
Additional information about Mount Thielsen is presented on the Cascade Volcano Observatory homepage of the U.S. Geological Survey.
Williams, H., 1933, Mount Thielsen, a dissected Cascade volcano: University of California Publications, Bulletin of the Department of Science, 23, p. 195-214.
Wood, C.A., and Kienle, J., 1993, Volcanoes of North America: Cambridge University Press, New York, 354 p.
Images of Volcanoes
To VolcanoWorld